1. Field of the Invention
The present invention relates to an image-taking apparatus that includes an imaging device and an image-taking optical system, and generates image signals representing a subject image formed on the imaging device through the image-taking optical system.
2. Description of the Related Art
There have been proposed techniques for employing a light-emitting diode (LED) as a light source in place of a xenon tube (see Japanese Patent Application Publications No. 2005-128413 and No. 2005-134711 for example). Use of the LED eliminates the constraints of the time for recharging a main capacitor and thus, it is possible to emit shooting fill light at any desired time.
Generally, when subject luminance is insufficient in multiple exposures or continuous shooting, fill light is emitted in a pulse at every shooting time so that subject luminance for each shooting is secured. If an LED is used in this type of shooting, it is possible to emit a predetermined amount of fill light at any desired time, because the LED is free from the constraints of the time for recharging a main capacitor as mentioned above.
However, LEDs have such a problem that the amount of light emitted from an LED gradually decreases with the passage of light-emission time.
FIG. 1 is a diagram showing the relationship between the light-emission time of an LED and the mount of light emitted from the LED. FIG. 2 is a diagram showing a gradual decrease in the amount of light emitted from the LED in continuous shooting. In FIGS. 1 and 2, the lateral axis indicates “time” while the vertical indicates the “amount of light” emitted from the LED.
LED has such a feature that its light-emission efficiency decreases as its temperature increases. Therefore, as shown in FIG. 1, the amount of light from the LED gradually decreases over the light-emission time. When light emission of an LED having such a tendency is used for continuous shooting or multiple exposures, there arises not only a problem that the amount of light emitted at one shooting gradually decreases over the time as shown in FIG. 1, but also another problem that the amount of light emitted at each shooting time during continuous shooting gradually decreases as the number of shot images increases as shown in FIG. 2. When the amount of emitted light gradually decreases in this way, a shortfall in the amount of light supplied to an imaging device becomes greater as the number of shot images increases. As a result, the brightness of images shot later becomes lower than that of images shot earlier among images shot during continuous shooting.
Japanese Patent Application Publications No. 2005-128413 and No. 2005-134711 provide multiple LEDs and divide them into several groups. The LED groups are caused to emit light at different shooting times in order to suppress the increase in temperature of the LEDs. However, incorporation of multiple LEDs in an image-taking apparatus increases the size of the image-taking apparatus. Recent LEDs can emit a relatively large amount of light as compared to LEDs in the past, but it is still smaller than the amount of light emitted from of xenon tubes. Use of such LEDs capable of emitting a larger amount light makes it possible to decrease the size of an image-taking apparatus by decreasing the number of LEDs.
Meanwhile, Japanese Patent Application Publication No. 2001-358988 discloses a technique for emitting fill light after increasing sensitivity by changing the gain in Automatic Gain Control (AGC) when a main capacitor has not yet been fully recharged. However, the technique disclosed in this document relates to a xenon tube, and thus it has nothing to do with an LED.